1. Field Of The Invention
This invention relates generally to techniques for the identification and analysis of contaminant particles on semiconductor wafers and, more particularly, to techniques for locating particles on notched wafers when using a high-magnification imaging device.
2. Description Of The Related Art
Semiconductor fabrication technology today deals with wafer sizes up to 200 mm (millimeters) and feature geometries with dimensions well below 1 .mu.m (micrometer). The presence of a contaminant particle larger than half the width of a conductive line on a wafer can lead to failure of a semiconductor chip made from the wafer. Therefore, a critical task facing semiconductor process engineers is to identify and, as far as possible, to eliminate sources of surface contamination.
Particle analysis on notched silicon wafers consists of locating contaminant particles on a wafer surface and analyzing their chemical compositions to determine the source of contamination. A well-known approach is to use a scanning electron microscope (SEM) in conjunction with analysis tool such as X-ray spectroscopy to respectively locate and analyze the particles. Because common particle dimensions are on the order of 0.1-1 .mu.m, however, initial magnifications on the order of 200 times to 2000 times are required for the particle to be seen on the SEM screen. At these high levels of magnification, only a small portion of the wafer is visible on the SEM screen at the same time, and, therefore, particle positions must be known quite accurately before the SEM can be used.
Because the SEM must be operated at high levels of magnification, it is not a useful instrument to obtain an overview of particles on an entire wafer. Other devices have been developed for this purpose and a two-stage process for locating and analyzing particles is generally employed. In the first stage, a laser scanning device raster-scans the wafer with a laser beam to locate most of the particles on the wafer. An example of such a device is a Tencor SurfScan 5000, manufactured by Tencor Instruments. The laser scanner then creates a laser scan map of the coordinates of the wafer features and contaminant particles. The manner in which the laser beam is scattered from the wafer features and the particles yields signals from which estimated particle positions in terms of x and y coordinates can be determined. However, because the scattering mechanism is not completely understood, the signals are of little help in identifying the type, chemical composition, and possible source of contaminant particles. This information can only be obtained with the help of a high-magnification imaging device, such as an SEM.
A critical aspect of this two-stage particle analysis method is that the coordinate system used in the laser scanning device must be transformed to the coordinate system used in the SEM or other similar imaging device. The notched wafer is physically moved from one device to the other, and there is no way to guarantee that the coordinate system used in the laser scanning device will still apply when the wafer is moved to the SEM. The wafer may be rotated inadvertently when it is moved from one device to the other, and the origins or zero reference points of the two coordinate systems will, in general, not be identical. Therefore, the particle coordinates obtained from the laser scanning device must be transformed to corresponding coordinates used in the SEM.
In the past, this transformation was accomplished by identifying, in both devices, two reference contaminant particles that are relatively recognizable due to their size and contrast. Given the coordinates of these two reference contaminant particles, as measured in the coordinate systems of both devices, a simple and well known coordinate transformation can be used to transform the remaining particle coordinates from one coordinate system to the other. See, for example, commonly assigned U.S. patent application, Ser. No. 07/886,541 to Uritsky et al. In general, transformation between two coordinate systems can be completely defined by an offset value and a rotation angle. That is to say, the transformation from one coordinate system to another can be considered to include a linear movement of the x and y axes so that the new origin assumes a position displaced from the old origin, together with a rotation of the axes about the origin to a new angular orientation. Such coordinate transformations are well known, and simple equations for performing them may be found in almost any basic text on linear algebra, coordinate geometry, or related subject matter. For example, transformations are completely defined in a text by John J. Craig entitled "Introduction to Robotics: Mechanics and Control," 2nd edition, published by Addison-Wesley Publishing Company. Inc. (1989), and specifically on pages 25-30. The transformation equations can also be found in "Elementary Linear Algebra," by Howard Anton, pp. 229-30, published by Anton textbooks, Inc. (1987).
Notched eight-inch (200 mm) wafers pose a particular challenge to the coordinate transformation problem due to their large size and limited reference marks. In general, the notch in notched eight inch wafers is only 1 mm.times.1 mm, or only 0.5% of the length of the 200 mm wafer diameter. Although a notch is typically a triangular indent in the circular wafer edge and having a rounded point, other types of indents and indented patterns are included within the meaning of the term. Flatted wafers are generally easier to analyze than notched wafers because the flat water includes a convenient and relatively long reference line with respect to which the wafer can be oriented. In the case of the notched eight inch wafer, however, there is no such reference line.
Several analysis methods which can facilitate the coordinate transformation for notched wafers have been previously considered. One solution is to set up the coordinate system of the Slim to be identical to that of the laser scanning device. Although this method appears feasible in theory, it is difficult and costly to implement in practice. Some form of precision mechanical fixture is necessary to hold the wafer in precisely the same orientation in both coordinate systems. Unfortunately, attempts to orient the notched wafer identically in both the SEM and the laser scanning device are prone to rotational error. Moreover, due to the large diameter of the notched wafer, a one degree error in orientation will result in a 1,700 .mu.m error (Error=r.theta.) for locating particles near the wafer edge.
A second approach is to orient the wafer such that the area near the notch is easily located in the SEM screen. The operator can then locate two reference particles near the notch that correspond to two particles on the laser scan map. Knowing the coordinates of the reference particles in both the SEM and laser scanning coordinate systems, a transformation can be applied to the laser scanned particle coordinates to yield SEM coordinates. However, particles may not always be present near the notch. Moreover, even if marks were physically generated near the notch, a difficulty tends to arise with the coordinate transformation itself since using two closely spaced points to compute the coordinate transformation can result in significant round-off error.
A final approach is to physically create known reference marks on the wafer surface, or to have known reference marks on the wafer prior to processing. However, to accurately define the transformation between coordinate system, the reference marks must be placed relatively far apart. Then, to find these marks using the SEM, a mechanical fixture would again be necessary to provide an accurate initial orientation. In addition, user generated marks damage the wafer and can cause additional contaminant particles to be generated.
From the foregoing discussion, it can be appreciated that there is a need for an improved method for transforming the coordinates of the laser scanning device to the coordinates of the SEM to accurately locate contaminant particles on an eight inch notched wafer. The present invention fulfills this need.